1. Field of the Invention
The invention relates to a mobile radio system comprising at least a transmitter which includes a controllable power amplifier whose output signal is fed back to a control input of the power amplifier via a detector device which produces a detector signal for detecting the output power of the power amplifier, and includes a signal generator for producing a control signal which is used for forming the output signal and has rising and falling edges.
The invention likewise relates to a transmitter used in such a mobile radio system.
2. Description of the Prior Art
In telecommunication systems in the time-division multiplex mode the variation with time of the output power of the power amplifier or the transmitted power of the transmitter is to be controlled or adjusted respectively, during a period of transmission. For example, the increase of transmitted power to a suitable power level and the final decrease of the transmitted power is to satisfy specific requirements, so that the interference to adjacent transmission channels is minimized.
More specifically, in mobile radio systems according to the GSM standard operating in the TDMA (Time-Division Multiple Access) mode, a mask is defined for the variation with time of the transmitted power during a time slot, which mask defines the permissible range boundaries of the level of the transmitted power in various time domains of a time slot. For example, the transmitted power is to be in a level range of 2 dB in width during data transmission. Furthermore, masks are defined for the various switching spectra of the various power levels to avoid interference to adjacent channels. In this manner limits are set to the variation with time of the increase and decrease of the transmitted power.
From Japanese published unexamined Patent Application 4-192636(A) is known a transmitter with a control loop comprising a controllable power amplifier, a detector device, a comparator (operational amplifier), a trapezoidal signal generating circuit and a reference voltage generating circuit. The detector device used for detecting the output power of the power amplifier, applies a detector signal to a first input of the comparator. The other input of the comparator is connected to the circuit generating the trapezoidal signals. The peak level of the trapezoidal signals is determined by the reference voltage. The output signal of the comparator is applied to the control input of the power amplifier.
The arrangement described here is devised as a closed control loop for the total duration of the time slot i.e. for the total duration of a transmission, i.e. the detector device is to be devised for all the level ranges of the transmitted power. In mobile radio systems according to the GSM standard the transmitted power covers a range of more than 70 dB. Detectors always producing an adequate detector signal in such a widespread level range demand much circuitry. Customary detector devices are generally incapable of detecting the output power of the power amplifier especially in the lower range of power levels. In such a case the control circuit described in said Japanese unexamined Patent Application is no longer closed and its function no longer guaranteed.